Water availability and scarcity are critical issues in many parts of the world. The growth of desalination technologies in recent years has made it possible to convert brackish water and seawater into potable water at competitive prices, offering the promise of alleviating some water scarcity issues. Unfortunately, desalination of brackish water is constrained by solution chemistry. Many natural brackish water sources that could be used as a water supply in water-scarce regions are chemically different from seawater. While sodium and chloride are the predominant cation and anion, respectively, in seawater, the major cations in many brackish waters are calcium or magnesium and the major anions are sulfate or bicarbonate. Calcium and magnesium salts of sulfate and carbonate are considerably less soluble than sodium chloride. As water is extracted via desalination, these cations and anions are concentrated until solubility limitations are reached, at which point precipitation occurs. The onset of precipitation has negative consequences for desalination systems primarily because precipitation happens on surfaces. In membrane-based desalination systems such as reverse osmosis (RO), scale formation on membrane surfaces rapidly degrades system performance by restricting the flow of water and ultimately damages the membranes. In thermally-driven desalination systems, scale formation rapidly degrades system performance by reducing heat transfer efficiency and clogging system components such as valves, pipes and heat exchangers.
For brackish water treatment, measures to prevent scale formation can be a dominating constraint in overall desalination system design. Desalination systems nearly always include subsystems for pH adjustment and antiscalant addition to minimize scale formation. The biggest constraint, however, is that overall water recovery by the process must be limited to levels that will minimize scale formation. As potable water is recovered, the cations and anions are concentrated in the concentrate stream, so a high degree of water recovery is accompanied by high concentrations of cations and anions and therefore high potential for scale formation. Furthermore, even when proper design and operating procedures are used, variations in source water quality or other changes in operation may result in scale formation and the consequent loss of system performance, damage to membranes, and clogging of system components. Thus, design must be conservative. For brackish water treatment, net potable water recovery of 70 to 85 percent of the feed water is common.
Limitations imposed on water recovery by solubility considerations have two direct negative consequences on brackish water desalination. First, desalination is practiced because of scarcity of fresh water, and in inland areas brackish water may also be scarce. Thus, desalination in water-scarce areas is accompanied by a need to recover as much water as possible. Low recovery is simply a loss (or poor utilization) of available resources. Second, the fraction of source water not recovered for beneficial use becomes a waste stream that contains the concentrated cations and anions. Disposal of this waste stream is expensive and problematic because of the potential for environmental contamination. The larger the waste stream, the larger the costs associated with waste management. In some desalination systems, waste management is the largest component of overall system costs.
Clearly, then, technology to prevent scale formation would allow greater water recovery, increased utilization of available water resources and decreased waste production. According to various embodiments, the invention described herein is intended to do exactly that.